Basic knowledge of a 3-phase induction motor

 

What should I do..? I have to connect a 3-phase induction motor to a 3-phase power source..

 

Did you call me?

 

Well.. yeah, yes. Actually, I’ve just read a book about electrical engineering because I have to connect a motor to a 3-phase power source. According to the book, I have two options to connect a motor. I showed the options in the chart below.

 

 

However, I couldn’t even understand what the chart above is showing..

 

I see. I will help you. By the way, do you have some basic knowledge about induction motors?

 

I can’t say I have..

 

That’s okay. So firstly, I think I should talk about what an induction motor is.

Here is a 3-phase induction motor that has 3 wires.

 

 

Ah, just a minute. I’ve seen a large induction motor that has six wires, not three wires.

 

 

I bet. I’m going to explain the difference between them in the last part of this lecture.

For now, I’d like you to pay attention to a motor that has 6 wires because that can help you understand the fundamental wiring way. So, let’s look inside the motor that has 6 wires.

 

 

Do you know what the three red rectangles mean in the chart above?

 

Well… I have no idea.

 

That’s absolutely fine. I’m going to tell you about them one by one. They are called “impedances”. Have you heard of it?

 

 

I think I have but I don’t know what impedance means at all…

 

I see. If that is the case, you can recognize the impedances as just resistances in this lecture. But in reality, they’re NOT resistances.

 

 

What do you mean??

 

You can regard that the resistances inside the motor consume energy that a power source is supplying so that the motor can rotate its shaft.

 

 

Is it like a light bulb can turn on by consuming energy from a battery?

 

That’s a good example to explain the situation. The bulb has a resistance in it and it consumes energy from the battery so that the bulb turns ON. A motor can rotate on the same principle.

 

I see.. But you told me that an impedance is different from a resistance, didn’t you?

 

Yes, I told you so but take it easy. I’m not going to go into the details about that because that is out of our topic today.

The important thing in this lecture is that electric current flows through the three red rectangles in the chart below so that the motor can rotate its shaft.

 

 

I understood.

 

 

Which part of a motor do the electrical symbols of a star and a delta exactly mean?

 

Let’s go into the topic about the difference between a star connection and a delta connection.

 

I don’t understand what the charts below mean at all. How are they related to a 3-phase induction motor?

 

 

That’s an easy question to answer. Please take a look at the three red wires in the chart below. Do you agree that the three wires are the same thing?

 

 

Yes, because all of the three red wires start with U and they go through a resistance, which strictly speaking is an impedance, and then the wires end with X. I can say that all the three wires are the same thing.

 

That’s right. So now, you can easily understand that the red wires below are the same thing because of the same reason.

 

 

Yes, I can. And then, the last remaining wires in the char below are the same thing as well with the same reason, right?

 

 

Good.

 

 

Actual wiring example

 

Now I understood the meaning of the symbols of a star and a delta but how can I actually connect the wires coming out of a motor to a 3-phase power source?

 

 

I have another question. Why do we need the two ways of star and delta? What would happen if our options were only star connection or only delta connection?

 

Good questions. I’ll answer all of your questions in this lecture.

Firstly, I’d like you to actually connect a motor using a delta connection. I’ll give you a hint. Can you do wiring and make the red point in the chart below?

 

 

I think I can. The point connects R, U, and Z. So, I’ll just connect all of the wires. Would you confirm the chart below?

 

 

Yes, that’s correct. You connected R, U, and Z. Will you make the point in the chart below next?

 

 

Sure. The point connects S, V, and X. So I’ll connect all of the wires. Please take a look at the chart below.

 

 

Thanks. Will you make the last point in the same way?

 

Yes it’s easy. Here it is.

 

 

Actually, you have already succeeded in connecting the motor with a delta connection. You have become able to connect a motor using delta connection.

 

Really? It was easier than I expected.

 

Let’s move on to our next mission. I’d like you to connect a motor using a star connection this time.

 

Sure. I think I can use the same method. Firstly, I’m going to make the point in the chart below.

 

 

The point connects X, Y, and Z. I’ll do wiring so that I can make the same point. Here is the chart. Am I right?

 

 

Yes, you are correct. Will you connect the remaining wires?

 

Yup. The chart below is the one I did.

 

 

Perfect. Do you have any questions so far?

 

 

How to make use of a star connection and a delta connection

 

No, I don’t. But I still don’t know the reason why the two ways of star and delta are necessary.. Both of them can rotate a motor, right?

 

Yes they can but as you can guess, they have their own characteristics.

 

I see. Would you explain the characteristics?

 

To put it simply, star connection can suppress the current value, and so the output of the motor goes down accordingly. Delta connection is the opposite. The amount of electric current in a delta connection is larger than that in a star connection, and so the output of the motor goes up accordingly.

 

 

What makes the difference? When should I use a star connection and when should I use a delta connection?

 

Let me tell you about what makes the difference in ” Why can a motor with a star connection keep the current value lower than a delta connection? “. I’m going to talk about how we should make use of star connection and delta connection in this lecture.

Since the output of a motor with a delta connection is larger than a star connection, a delta connection is advantageous for machines. That’s why a delta connection is commonly used.

 

 

However, a delta connection may cause a problem.

 

What kind of problem may occur with delta connection?

 

When a motor is trying to start rotating, the motor needs a lot of energy since the motor is trying to rapidly move an object at rest. So, a lot of electric current flows through a large motor with a delta connection.

 

 

If that is the case, how can we start a large motor?

 

You should start a motor with a star connection. Do you remember that a star connection can suppress the current value?

 

Yes, I do. But you told me that a motor with a star connection is disadvantageous for machines because the output of the motor becomes smaller…

 

It’ll be fine if you start the motor with a star connection and after the rotation speed goes up enough, the control system of the machine automatically switches the connection from star to delta.

 

How can a control system switch the connection from star to delta?

 

I’m afraid but that’s out of our topic today. May be some other time.

 

May I give you another question?

 

What is it?

 

 

Why do some motors have 3 wires and others have 6 wires?

 

Is it okay if I start a small motor with a delta connection?

 

Yes. It’s no problem.

 

Doesn‘t overlarge electric current flow through the small motor?

 

Of course the theory of star and delta is applied to a small motor as well, but overlarge electric current can’t flow through a small motor. What I meant is that the motor is so small that overlarge electric current can’t flow through the motor.

 

I understood. A small motor needs only a delta connection.

 

Yes. And that’s why a small motor has only 3 wires, not 6 wires. Do you know the reason?

 

Well… Would you tell me the details about that?

 

I prepared a motor below and the motor has already been wired with a delta connection.

 

 

If the motor is small, it only needs a delta connection, right?

 

Yes because we can start a small motor with a delta connection.

 

That’s why the wires of a small motor was already connected inside the motor like the chart below before the shipment of the motor.

 

 

Yeah, I can see only three wires coming out of the motor. That’s why some motors have only three wires.

 

I think that you understood the difference between a delta connection and a star connection.

 

Yes. Thank you so much.